A Darlington transistor circuit, sometimes referred to as a Darlington amplifier or a Darlington pair, is a compound transistor circuit which consists of two transistors in which the collectors are tied together and the emitter of the first or input transistor is directly coupled to the base of the second or output transistor. The emitter current of the input transistor equals the base current of the output transistor. Small current or logic level signals are applied to the base of the input transistor to control high currents in the collector emitter load circuit of the output transistor. The Darlington transistor circuit has the advantage of a much higher current gain compared to a single transistor.
When utilizing the Darlington transistor for switching reactive loads, the removal or reversal of the positive bias on the base of the input transistor shuts off the circuit, while at the same time the reactive load is forcing high power through the output transistor. The load which is in series with the supply voltage tends to force the voltage up to the sustaining or breakover voltage of the output transistor. Reaching this voltage level at high current tends to cause secondary breakdown in the output transistor, and limits the power handling capability of the circuit. When the inductive portion of the load forces the voltage to reach the sustaining voltage of the output transistor, either the transistors are destroyed, if the inductance level of the load is great enough, or such inductance may prolong the turnoff time of the transistor circuit. Thus, the power handling capabilities of the circuit are usually maximized at a voltage level below the sustaining voltage of the transistors.
Heretofore, the power handling capability of the Darlington amplifier was increased by connecting a Zener diode across the collector-base of the input transistor, for example, or across the collector-base of the output transistor, is shown in U.S. Pat. No. 3,435,295, so that the reactive load causes the Zener diode to breakover at the Zener voltage, thereby establishing a maximum voltage across the output transistor.
Other solutions to the problem of secondary breakdown have been proposed; for example, connecting a first resistor between a potential source on the direct connection of the input and output transistor; and a second resistor between the collector electrodes of the input and output transistors, as shown in U.S. Pat. No. 3,210,561. Also it has been proposed to provide an integrating element whose charging and discharging circuits are decoupled; and which is arranged between the collector and base of the transistor in order to permit controlled charging of the integrator when the inductor is first disconnected.
The purpose of the present invention is to provide a Darlington transistor circuit which retains the high gain feature while at the same time substantially increasing the reverse bias safe operating area of the transistor circuit to handle increased loads without requiring extra components to protect the transistor circuit.